A present inverter includes a combination of MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) which is a semiconductor device with a switching function and a diode which is a semiconductor device with a rectification function. That is, a switching element and a rectifier are paired with each other to be basically used in the inverter. Furthermore, two or more pairs of the switching element and the rectifier are combined with each other to configure the inverter entirely. However, many elements included in the inverter leads to its cost increase and low reliability.
There has been an attempt to provide a single MOSFET with both the switching function and the rectifying function in a technical field of a switching element, specifically an insulated gate transistor like a MOSFET. Such a single MOSFET includes a p-type region in contact with a source region, an n-type region in contact with a drain region, and a low-concentration n-type drift layer formed between the p-type and n-type regions to form a PiN diode.
However, a PiN diode has a high rising voltage and performs bipolar operation to cause a high switching loss. There is another attempt to replace a built-in diode region of the MOSFET with a Schottky barrier diode (SBD) for lowering a rising voltage and a stationary loss of the MOSFET. The SBD can also reduce the switching loss of the MOSFET owing to unipolar operation of an SBD. The built-in diode region includes the PiN diode having a high rising voltage. However, when an SBD is formed adjacent to a MOSFET to provide a built-in SBD, a unit cell becomes large in size because the unit cell includes the SBD region also, thereby making it difficult to handle large current capacity necessary for an inverter. In addition, an SBD commonly has a low breakdown voltage depending on the shape or position of the SBD in a unit cell, thereby easily causing the breakdown of the SBD whenever a reverse voltage is applied to the SBD.
So, an idea is proposed as follows. Firstly, a trench is formed between adjacent gate electrodes of MOSFETs so that the trench passes through a channel layer between the adjacent gate electrodes. Secondly, a Schottky metal layer is formed inside the trench to configure a built-in SBD in the bottom area of the trench. As a result, the built-in SBD is formed in a diffusion region of the MOSFETs. However, when the Schottky barrier of the SBD is in contact with a drift layer near the drain region, the Schottky barrier is easily subjected to electric field concentration, thereby causing a breakdown of the SBD in some cases before a voltage applied to the SBD reaches the breakdown voltage of the SBD.
In addition, a vertical SiC MOSFET having a built-in SBD is proposed as follows. The material of the SiC MOSFET is SiC (Silicon Carbide). The SBD with a low on-resistance is formed adjacent to the MOSFET by forming a metal electrode on an exposed surface area of the n-type drift layer of the MOSFET to configure the Schottky barrier. However, the unit cell becomes larger in size just for adding the SBD than a unit cell of a previous MOSFET without a built-in SBD. For this reason, the ratio of the area occupied by the vertical MOSFET decreases to cause an increase in the on-resistance of the vertical MOSFET. Such a size problem may be resolved by forming a region of the source electrode in a trench. Such a trench-shaped source electrode commonly employs a vertically laminated structure including an n+ source region highly doped with n-type dopants and a p+ source region highly doped with p-type dopants. Forming the trench-shaped source electrode in electrically contact with both the n+ and p+ source regions shortens the distance between the Schottky junction interface of the SBD and the bottom of the p type well, thereby reducing an electric-field relaxing effect of the p-type well and causing the deterioration of a breakdown voltage. Here, the Schottky junction uses the source electrode as a Schottky barrier metal.